The present invention relates to an apparatus for use in networks which connect a plurality of computers, and more particularly, to an apparatus for transferring digital data between time-domain multiplexed communication links of a telephone system that forms part of said computer network.
Many businesses have computer networks that extend over large geographic areas. The most economical manner for implementing such networks utilizes existing telephone lines to connect the various computers in the network. Computer networks that utilize the telephone system are known to the prior art. However, these computer networks are less than ideal. Problems arise from the use of transmission facilities that were designed for voice quality transmissions between two points, and from the inclusion of components that are owned and maintained by separate commercial entities. However, because of the cost savings inherent in the use of the existing telephone system, it is desirable, if at all possible, to adapt such network equipment for acceptable digital data transmission.
Existing computer networks suffer from a number of drawbacks. For example, prior art computer networks that utilize the telephone system are difficult to reconfigure both with regard to the speed at which data is transmitted within the telephone system and with regard to the topological configuration of the computer network. In prior art computer networks, telephone company technicians must be dispatched to various locations within the telephone system to effect a data rate change in the computer network. If the new data rate is not one of a small number of rates for which the equipment is designed, the actual interface circuitry within the telephone system must often be changed. As a result, it is practically impossible to change data rates in response to short-term changes in the quality of the telephone lines carrying the data.
Short-term increases in the noise on specific telephone lines are common. The telephone lines used for carrying the user's computer data are, in general, identical to the lines used to carry voice communications within the telephone system. These lines are bundled into cables with similar lines. The noise level on these lines depends upon a number of factors including the nature of the transmissions on the lines bundled with those used for the computer network.
To change the network speed or configuration, changes must be made in equipment that is neither owned nor maintained by the computer network owner. This makes reconfiguration of the network difficult, since telephone company personnel must be coordinated with those of the network owner.
Ideally, a network owner would like to be able to change the configuration of the network in response to observed error rates or the time of day. Such reconfiguration, for example, would be useful in smoothing peak loads on the computer network. Such reconfigurability would also be useful in transferring specific services to offices in earlier time zones when the office providing the service in another time zone closes at the end of the day. In prior art systems, such reconfiguration requires that key components of the computer network be rewired by telephone company personnel or that special purpose hardware be installed at significant cost.
Another example of a drawback in prior art computer networks is the inability of the user to monitor and manage the computer network. If a failure occurs in the computer network, it is difficult to ascertain whether the failure is the result of a malfunction in the equipment supplied by the telephone company, either on or off of the network owner's premises, or in the equipment owned and maintained by the network owner. Hence, when a malfunction is detected, telephone company technicians are often needed to ascertain the location of the failure within the system. The time delays inherent in such determinations are often unacceptable. It would be advantageous to provide a means for allowing the network user to set loopbacks and other diagnostic aids within the computer network without the time delays and added expense inherent when telephone company personnel must be involved. Similarly, the computer network user would like to be able to access data showing error rates on the various communication links in the computer network.
Within the telephone system, the computer data is often communicated on time-domain multiplexed communication links referred to as T.sub.1 lines. Each T.sub.1 line provides 24 time-domain multiplexed channels. In moving data between geographically distant points within the computer network, data must often be transferred from a first channel on one T.sub.1 line to a second channel on a different T.sub.1 line. This data transfer function is normally performed using the digital cross-connect switches which are located in various central offices within the telephone system.
Unfortunately, not all central offices have such cross-connect switches. When a computer network must shift data between different T.sub.1 channels within such a central office, special T.sub.1 interface circuits are utilized. These circuits allow data from a first T.sub.1 channel to be transferred to and from a second T.sub.1 channel either on the same T.sub.1 line or on a different T.sub.1 line. The present invention relates to such an interface circuit.
As noted above, error rates are a significant problem in computer networks that utilize voice quality communication links within the telephone system.
It has been found that one method of reducing errors on the various communication links within the computer network is to encrypt the data using an error correcting code prior to transmission. Hence, it would be advantageous to provide a switching unit that would function properly on error encrypted data.
Broadly, it is an object of the present invention to provide an improved T.sub.1 interface circuit.
It is another object of the present invention to provide a T.sub.1 interface circuit that allows the use of encrypted data to achieve an improved error rate.
It is yet another object of the present invention to permit modification of the topological configuration of the computer network without the necessity of rewiring or reconfiguring network hardware.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.